1. Field of the Invention
This invention relates to a power supply device comprising a plurality of resonance type switching converters connected in parallel.
2. Prior Art
Current resonance type switching converters are known. A current resonance type switching converter is realized by adding a resonance circuit to a switching transistor for the purpose of switching voltages. Current resonance type switching converters provide a major advantage that the electric current flowing through the primary side of the transformer shows a sinusoidal waveform to reduce the current loss and the switching noise due to switching operations because of the resonance circuit added to it.
Power supply devices realized by connecting a plurality of such current resonance type switching converters in parallel are also known. It is possible to supply a large power to a load when a plurality of current resonance type switching converters are connected in parallel.
FIG. 1 of the accompanying drawings shows a schematic circuit diagram of a known power supply device realized by connecting a pair of current resonance type switching converters.
The known power supply device 100 of FIG. 1 comprises an AC input terminal 102, a power factor improving circuit (P. F. C.) 103, a first current resonance type switching converter 104, a second current resonance type switching converter 105, a frequency control circuit 106 and a feedback circuit 107.
Typically a commercial AC voltage may be applied to the known power supply device 100 by way of the AC input terminal 102. The applied AC voltage is then fed to the power factor improving circuit 103. After boosting the applied AC voltage and improving its power factor, the power factor improving circuit 103 rectifies the AC and outputs a DC input voltage (Vin) that may be for instance as high as 380(V).
The DC input voltage (Vin) is then fed to the first current resonance type switching converter 104 and the second current resonance type switching converter 105. Note that, hereinafter, the first current resonance type switching converter 104 and the second current resonance type switching converter 105 are referred to simply as the first converter 104 and the second converter 105 respectively.
The first converter 104 is controlled for its switching frequency by a frequency control signal from frequency control circuit 106 and converts the DC input voltage (Vin) it receives into a DC output voltage (Vout) that is stabilized to show a predetermined voltage value. Similarly, the second converter 105 is also controlled for its switching frequency by a frequency control signal from the frequency control circuit 106 and converts the DC input voltage (Vin) it receives into a DC voltage (Vout) that is stabilized to show a predetermined voltage value. The output terminal of the first converter 104 and that of the second converter 105 are connected in parallel and are used to supply the load 101 with their respective DC outputs (Vout).
The first converter 104 is provided with a DC input terminal 111 and the DC input voltage (Vin) is applied to it by way of the input terminal 111. The first converter 104 is also provided with a control signal input terminal 112 and a frequency control signal is applied to it from the frequency control circuit 106 by way of the control signal input terminal 112.
The first converter 104 has a first switching transistor 113 and a second switching transistor 114. The collector of the first switching transistor 113 is connected to said DC input terminal 111. The collector of the second switching transistor 114 is connected to the emitter of the first switching transistor 113, which emitter is then grounded.
The first converter 104 also has a driving transformer 115 for driving the first switching transistor 113 and the second switching transistor 114.
The driving transformer 115 includes a primary winding 115a and a pair of secondary windings 115b, 115c. The primary winding 115a of the driving transformer 115 is fed with the frequency control signal sent from the frequency control circuit 106 by way of the control signal input terminal 112. The two secondary windings 115b, 115c of the driving transformer 115 are wound in opposite directions. One of the secondary windings, or the secondary winding 115b, is connected at an end thereof to the base of the first switching transistor 113 by way of resistor 116 and at the other end thereof to the emitter of the switching transistor 113. On the other hand, the other secondary winding, or the secondary winding 115c, is connected at an end thereof to the base of the second switching transistor 114 and at the other end thereof to the emitter of the second switching transistor 114.
The first switching transistor 113 and the second switching transistor 114 connected respectively to the two secondary windings 115b, 115c of the driving transformer 115 that are wound in opposite directions are complementarily switched according to the frequency control signal input to the primary winding 115a of the driving transformer 115.
The first converter 104 includes an insulating transformer 117, a resonance capacitor 118 arranged at the primary side of the insulating transformer 117 and first and second rectifier diodes 121 and 122 arranged at the secondary side of the insulating transformer 117.
The primary winding 117a of the insulating transformer 117 is connected at an end thereof to the emitter of the first switching transistor 113 and grounded at the other end thereof by way of the resonance capacitor 118 and resistor 120. The secondary winding 117b of the insulating transformer 117 is connected at an end thereof to the anode of the first rectifier diode 121 and at the other end thereof to the anode of the second rectifier diode 122. The cathode of the first rectifier diode 121 and that of the second rectifier diode 122 are connected to positive side output terminal 123, whereas negative side output terminal 124 is connected to the middle point of the secondary winding 117b of the insulating transformer 117.
When the first switching transistor 113 and the second switching transistor 114 of the first converter 104 having the above described configuration are complementarily and repeatedly turned on and off according to the frequency control signal from the frequency control circuit 106, a voltage having a rectangular waveform is applied to the opposite ends of the primary winding 117a of the insulating transformer 117. As a voltage having a rectangular waveform is applied, a resonance current having a sinusoidal waveform flows through the primary winding 117a as a concerted effect of the capacitance of the resonance capacitor 118 and the inductance of the insulating transformer 117. It should be noted that a resonance current flows through the primary winding 117a of the insulating transformer 117 only when such a voltage is applied to the opposite ends of thereof. As a resonance current flows through the primary winding 117a, the energy applied to the primary winding 117a is transferred to the secondary winding 117b to cause an electric current to flow therethrough. The electric current flowing through the secondary winding 117b is rectified by the two rectifier diodes 121, 122 and output from the positive side output terminal 123.
Load 101 is connected between the positive side output terminal 123 and the negative side output terminal 124 of the first converter 104 having the above described configuration. Additionally, a smoothing capacitor 125 is arranged between the positive side output terminal 123 and the negative side output terminal 124 so that a stabilized DC output current (Vout) is fed to the load 101 from the first converter 104 with a predetermined voltage value.
The second converter 105 has a configuration same as that of the first converter 104.
A DC input voltage (Vin) is applied to the DC input terminal 111 of the second converter 105 and a frequency control signal is fed from the frequency control circuit 106 to the control signal input terminal 112 of the second converter 105.
The load 101 is also connected between the positive side output terminal 123 and the negative side output terminal 124 of the second converter 105 having the above described configuration. Additionally, a smoothing capacitor 125 is arranged between the positive side output terminal 123 and the negative side output terminal 124 so that a stabilized DC output current (Vout) is fed to the load 101 from the second converter 105 with a predetermined voltage value.
The feedback circuit 107 has a differential amplifier 131, a reference voltage source 132 and a photocoupler 133.
The inverting input terminal of the differential amplifier 131 is connected to the connection point of potential dividing resistors 136 and 137 for dividing the DC output voltage (Vout) applied to the load 101. The non-inverting input terminal of the differential amplifier 131 is connected to the reference voltage source 132 adapted to generate reference voltage (Vref).
The photocoupler 133 has a light emitting element which is a light emitting diode 134 and a light receiving element which is a phototransistor 135. The anode of the light emitting diode 134 of the photocoupler 133 is connected to the positive side output terminals 123 of the first and second converters 104 and 105. The cathode of the light emitting diode 134 of the photocoupler 133 is connected to the output terminal of the differential amplifier 131 by way of resistor 138. The emitter of the phototransistor 135 of the photocoupler 133 is grounded. The collector of the phototransistor 135 of the photocoupler 133 is connected to the feedback terminal of the frequency control circuit 106 by way of input resistors 141, 142 of the frequency control circuit 106.
Of the feedback circuit 107 having the above described configuration, the differential amplifier 131 detects the voltage obtained by dividing the DC output voltage (Vout) applied to the load 101 by means of the dividing resistors 136, 137 and the error voltage representing the difference between that voltage and the reference voltage (Vref) obtained from the reference voltage source 132. The error voltage is then input to the feedback terminal of the frequency control circuit 106 by way of the photocoupler 133.
The frequency control circuit 106 controls the oscillation frequency of the frequency control signal it supplies to the first and second converters 104 and 105 according to the error voltage input to the feedback terminal. More specifically, the frequency control circuit 106 raises the oscillation frequency of the frequency control signal when the voltage obtained by dividing the DC output voltage (Vout) by means of the dividing resistors 136, 137 is higher than the reference voltage (Vref). The frequency control circuit 106 lowers the oscillation frequency of the frequency control signal when the voltage obtained by dividing the DC output voltage (Vout) by means of the dividing resistors 136, 137 is lower than the reference voltage (Vref).
Note that the electric power transmitted to the secondary winding 117b of the insulating transformer 117 of the first converter 104 and the second converter 105 is expressed by "the voltage applied to the primary winding 117a".times. "resonance current flowing through the primary winding 117a". As described above, the resonance current flowing through the primary winding 117a of the insulating transformer 117 of either of the first and second converters 104, 105 shows a substantially sinusoidal waveform. Therefore, the power transmitted to the secondary winding 117b is raised when the switching frequency of the resonance current falls and lowered when the switching frequency of the resonance current rises. Thus, the power transmitted to the secondary winding 117b can be controlled by making the switching frequency of the resonance current variable.
In the above described known power supply device 100, the oscillation frequency of the frequency control signal is changed as a function of the DC output voltage (Vout) applied to the load 101 in a manner as described above.
To be more accurate, the feedback circuit 107 compares the DC output voltage (Vout) applied to the load 101 with the reference voltage (Vref) and, if the DC output voltage (Vout) falls below the reference voltage, it lowers the input voltage of the feedback terminal of the frequency control circuit 106, whereas, if the DC output voltage (Vout) rises above the reference voltage, it raises the input voltage of the feedback terminal of the frequency control circuit 106. Then, the frequency control circuit 106 raises the oscillation frequency of the frequency control signal to lower the DC voltage output (Vout) of the first converter 104 and the second converter 105 when the input voltage of the feedback terminal is high. On the other hand, the frequency control circuit 106 lowers the oscillation frequency of the frequency control signal to raise the DC voltage output (Vout) of the first converter 104 and the second converter 105 when the input voltage of the feedback terminal is low.
With the above described control operation, the known power supply device 100 can supply a stabilized DC voltage to the load 101.
Meanwhile, the first converter 104 and the second converter 105 of the above known power supply device 100 are switched for operation by a same frequency control signal in a manner as described above. In other words, they are operated with a same switching frequency. Thus, the resonance current can differ between the first converter 104 and the second converter 105 if there is any discrepancy between them in terms of the impedance of the insulating transformer 117, the impedance, the capacitance of the resonance capacitor 118, the coupling of the primary winding 117a and the secondary winding 117b of the insulating transformer 117 and the impedance of the secondary side of the insulating transformer 117. Since the voltage applied to the first converter 104 and the one applied to the second converter 105 have a same value, the power output is no longer equally shared by the two converters if the resonance current differs between the first and second converters 104 and 105.
The inductance of an insulating transformer normally fluctuates to an extent of about .+-.10 to 15%. Similarly, the capacitance of a capacitor normally fluctuates to an extent of about .+-.3 to 5%. Therefore, there arise a difference of about 20 to 30% between the first and second converters 104 and 105 in terms of their shares of the power output if only the fluctuations in the inductance of the insulating transformer and those in the capacitance of the capacitor are taken into consideration.
In a power supply device comprising a plurality of resonance type switching converters connected in parallel, if the power output is equally shared by the converters, those bearing the power output to a large extent have to be subjected to large stress at the cost of the service life their components and their reliability. Additionally, in the case of a power supply device showing a high power output level that exceeds 500 to 1,000W, the power output is made to change enormously relative to the switching frequency for the purpose of achieving a high efficiency. Thus, it is highly desirable that the converters of such a power supply device equally share the power output. If the first and second converters 104 and 105 of the above described power supply device 100 share the power output at a ratio of 7:3 and the share of the converter that normally bears 70% of the power output is raised abruptly, the device can shut down because the converter can no longer bear the excessive load.